Without limiting tile scope of the present invention in general, the background of the invention is described by way of example in its application to hearing aids.
A person's ability to hear speech and other sounds well enough to understand them is clearly important in employment and many other daily life activities. Improvements in hearing aids which are intended to compensate or ameliorate hearing deficiencies of hearing impaired persons are consequently important not only to these persons but also to the community at large.
Electronic hearing aids and methods are discussed in coassigned U.S. Pat. 4,548,082 by Engebretson (an inventor herein), Morley (an inventor herein) and Popelka, which is hereby also incorporated herein by reference as an example of an electronic system in which the present invention can be used.
An article on electronic hearing aid problems by one of the present inventors (Morley) is "Breaking the frequency barrier" IEEE Potentials, February 1987, pp. 32-35.
"Digital Filtering Using Logarithmic Arithmetic" by N. G. Kingsbury et al., Electronics Letters 7: 56-58 (1971) discusses multiplication by adding logarithms, and using a read-only-memory to do addition and subtraction. Logarithmic analog-to-digital and digital-analog conversion are mentioned.
"RC Logarithmic Analog-to-Digital (LAD) Conversion" by E. J. Duke, IEEE Transactions on Instrumentation and Measurement, February, 1971, utilizes an RC circuit approach to conversion.
"All-MOS Charge Redistribution Analog-to-Digital Conversion Techniques-Part II" by R. E. Suarez et al., IEEE J. Solid-State Circuits, vol. SC-10, pp. 379-385, December 1975, describes a two-capacitor successive approximation technique for linear conversion of each bit in a digital word.
Moser U.S. Pat. No. 4,187,413 describes a hearing aid with a finite impulse response (FIR) filter and states that it can be implemented using only one multiplier in a time multiplexed configuration.
Steager U.S. Pat. No. 4,508,940 suggests a hearing aid based on sampled-data analog circuits which has a plurality of parallel signal channels each including a bandpass filter, controlled gain amplifier with volume control, circuits for non-linear signal processing and bandpass filter.
Conventionally, a microphone in the hearing aid generates an electrical output from external sounds. An amplifying circuit in the aid provides a filtered version of the electrical output corresponding to the sounds picked up by the microphone. The filtering can be due to an inherent characteristic of the amplifying circuit or may be deliberately introduced. The amplified and filtered output of the hearing aid is fed to an electrically driven "receiver" for emitting sound into the ear of the user of the hearing aid. (In the hearing aid field, a receiver is the name of an electronic element analogous to a loudspeaker or other electroacoustic transducer.) Some of the sound emitted by the receiver returns to the microphone to add a feedback contribution to the electrical output of the microphone. The feedback is amplified by the hearing aid, and ringing or squealing often arise in an endlessly circular feedback process.
The commercial viability of a hearing aid having sophisticated performance features as described in coassigned U.S. Pat. No. 4,548,082 is strongly tied to the premise that electronic circuits can be realized in which the total power consumption does not exceed a few milliwatts. Given the computationally intensive nature of the signal processing required to implement filtering used in shaping the desired frequency selective response as well as the power needed by numerous circuits needed for signal conversion and amplification, this is an extremely difficult task. Only a few hundred microwatts are available for the filtering in a viable hearing aid.
Sophisticated performance requirements imply a massive computational burden. It is believed that millions of arithmetic calculations per second are probably needed in a sophisticated hearing aid. Generally speaking, electronic circuitry dissipates more power as performance increases, unless basic improvements can be found.